For an aircraft provided with an automatic pilot, for instance, a civil transport airplane, it is known that, during the approach stage during a landing operation controlled by the automatic pilot, a large number of guiding laws are implemented, within the automatic pilot itself. Such guiding laws make it possible to bring the aircraft up to the runway threshold and to implement the flared landing, the alignment, the nose drop and then the deceleration on the runway. To each guiding law, there is associated a particular automatic pilot mode, so that the engagement of the automatic pilot modes results in the corresponding guiding laws being activated. The good occurrence of a landing operation as controlled by the automatic pilot thus depends on the consistent sequencing and behavior of the different pilot modes.
It is moreover known that, during such an automatic approach phase (i.e. controlled by the automatic pilot), engaging the pilot modes by the automatic pilot depends, more particularly, on the current height of the aircraft with respect to the ground.
However, the current height of the aircraft is mainly determined by radio altimeters (generally two or three, on board the aircraft and connected to the automatic pilot thereof), being sensitive to external disturbances (for instance, clouds, rain, etc.). They could thus provide wrong data, likely to be misinterpreted by the automatic pilot and to disturb the engagement of the automatic pilot modes.
Now, an untimely engagement (or a lack of engagement) of an automatic pilot mode could result in an untimely activation (or a lack of activation) of a guiding law, leading to the aircraft being guided inappropriately.
For overcoming such drawbacks, it is known to check the consistency of the heights as provided by radio altimeters of the aircraft and, when they are inconsistent therebetween, to trigger an alarm in the cockpit so as to warn the crew a problem has occurred and that it should be preferred to interrupt the automatic approach. Such a monitoring generally carried out by the automatic pilot, is based on the fact that it is unlikely that the various altimeters of an aircraft should simultaneously emit, at a given moment, wrong data, but being consistent one relative to the other.
However, frequently, a warning is emitted and the automatic approach is interrupted even when the height being read by one of the radio altimeters is right, amongst the received inconsistent heights.
Moreover, in the case where only one of the radio altimeters is operating, comparing the heights provided by the different radio altimeters is not more feasible, so that, as a precaution measure, a warning is emitted and the automatic approach interrupted, although the height provided by the radio altimeter in operation could be right.